1. Field of the Invention
The present invention relates to a method and an arrangement for the determination of the optical properties of a multi-layered tissue. More specifically, the invention relates to a method for the detection and characterization of tumors in a tissue.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
New developments in imaging by means of visible and near-infrared light show promise in medical diagnosis, because light can offer three key benefits over traditional diagnostic tools based on e.g. X-rays, ultrasound, or nuclear magnetic resonance. First, light at different wavelengths interacts with tissue in distinctive ways and forms the basis for spectroscopy, which allows one to optimize the wavelength for a specific application. Second, image-processing methods are becoming powerful enough to make it possible to use just a few photons, and thus allow imaging based on low-level or noisy signals. Third, light offers a good compromise between the lower-resolution radio frequency or ultrasound imaging and the shorter wavelength, higher resolution, but harmful ionizing radiation of X-rays. Also, optical methods are usually non-invasive and non-toxic, and have the potential to be realized in terms of compact and inexpensive devices.
Any imaging or spectroscopic method must deal with both the absorption by the primary component of tissue, i.e. the aquatic solution, and the absorption and scattering by various types of tissue “particles”. Spectroscopy is used for measuring time-dependent total variations in the absorption and scattering in large volumes of tissue. For example, brain oxymetry (haemoglobin spectroscopy) can reveal internal bleeding caused by head injury. Imaging is important when it is of interest to detect a localized heterogeneity of the tissue, such as an early breast or brain tumor or a small amount of bleeding in the brain. Then imaging enables one to identify the site of the trauma and differentiate it from the surrounding tissue. Tumors represent a structural anomaly that one desires to detect, localize, and classify. Tumor growth is associated with (i) a larger blood volume resulting from a relatively larger number density and volume fraction of blood vessels in the tumor, (ii) increased concentrations of the intracellular organelles required for the energy production associated with rapid growth, and (iii) accumulation of highly scattering calcium precipitates. Some of these properties are expected to be helpful in classifying tumors as benign, malignant, and so on.
Light that enters tissue is absorbed and scattered by compounds in the tissue called chromophores. The major chromophores are melanin, haemoglobin, and cytochromes. Melanin is a pigment that colors our skin and protects us from sunburn. It attenuates UV light strongly by acting as a Rayleigh scatterer. Hemoglobin (Hb) is a colored pigment found in red blood cells. It is a large molecule that can bind oxygen molecules to form HbO2. Cytochromes consist of a series of enzymes found in the membrane of the mitochondria. They have absorption spectra that depend on whether the enzyme is in its oxidized or reduced state. Cytochromes can be monitored by optical means. For example, NADH is an important compound that absorbs strongly in the UV (310-375 nm). If NADH is exposed to UV light, it will fluoresce with a broadband emission spectrum around 460 nm. The absorption spectrum of Hb is different from that of HbO2. Thus, highly oxygenated arterial blood looks bright red, while venous blood containing more deoxygenated haemoglobin appears bluish red. The absorption coefficient varies with wavelength, but is typically 0.02-0.1 per millimeter in the visible and NIR parts of the spectrum. It also depends on chromophore content (especially the amount of blood). By tissue particle is meant a small volume of tissue with a complex refractive index that is different from that of the surrounding medium. The absorption by the aquatic component is known or can be measured for healthy tissue. A small volume surrounding a tumor is characterized by increased blood concentration and thus enhanced or anomalous absorption. Particles much smaller than the wavelength of light consisting of cell nuclei or mitochondria are called Rayleigh scatterers. Particles much larger than the wavelength of light consisting of cells or groups of cells are called Mie scatterers. The scattering coefficient varies with wavelength but is typically in the range 20-100 per millimeter.
Knowledge of the optical properties of biological tissue is the critical basis for carrying out studies in biomedical imaging as well as for developing instruments for medical diagnosis.
The physiological state of a biological tissue can be obtained from its absorption and scattering coefficients. By physiological state is meant the relative concentrations of aquatic and non-aquatic components as well as the chemical composition of non-aquatic tissue components including blood vessels, organelles, etc. Thus, it is desirable to develop accurate and reliable methods for determining optical properties of tissue.
Optical coherence tomography (OCT) has been successfully applied in biomedical imaging. This method relies on the use of coherent light, and it can be used to image the architectural morphology or glandular organization of tissues. However, its penetration depth is limited to 2-3 mm, and this technique, as currently practiced, does not provide the information needed to determine the optical properties of a biological system.
Since coherent light provides limited depth information, many current optical imaging techniques rely on the use of diffuse light, which carries information about deeper layers, i.e. about 4-8 mm within the tissue.
These methods include;                (i) time-resolved techniques for detection of so-called “snake” or “ballistic” photons that have propagated along nearly-straight paths        (ii) diffuse optical tomography, and        (iii) tomographic imaging using diffuse photon density waves created by intensity-modulating the incident light energy.        
In imaging with diffuse light, approaches to the solution of the radiative transfer problem are frequently based on the diffusion approximation to the radiative transfer equation.
By the term “radiative transfer problem” is meant transport of light in a multiple scattering medium such as tissue.
A review of the current literature is given in table 1.
PhysicalEquipmentCommercialMethodPrincipleStrengthLimitationNeedFeasibilityCommentsOCTbased onhigh spatialpenetration depthoptical gatingavailableperforms architecturalcoherent lightresolutionis limited& correlationmorphology, but is unable todevicesimage tissue optical propertiesLaser CT (1)based on CTcan determinepenetration depth isheterodyneunderbased on transmitted signal, cantheoryextinctionlimiteddetectioninvestigationonly be applied to a very thin layercoefficientdeviceof tissueTime-resolvedbased on ballisticcan determinepenetration depth ishigh time-promisinghigh time resolution is necessary,diffusephotonsopticallimitedresolutionbut is difficult to doimagingpropertiesdeviceCW diffusebased on diffusereal-timesignal to noiseCW laserunderperforms quantitative functionalopticalphotonsfunctionalproblemsourcesinvestigationbrain imaging, but does nottomography (2)imagingdetermine tissue optical propertiesTOAST (3)based on finitereal-timebased on time-pulsed laserundercontains the limitation inherent inelement modelfunctionalresolved& high time-investigationtime-resolved techniquesimagingmeasurementsresolutiondeviceDiffusebased onsimple & fastdetermines onlyCW laserunderit is insufficient to use onlyopticaldiffusion theoryabsorption coefficientsourceinvestigationabsorption coefficient to describe areflectionbiological tissuetomography(4)DPDWbased on diffuseanalyticaltransmitted DPDWlight sourceundercannot determine opticaltomographyphoton densitysolution forsignal is weakmodulationinvestigationproperties of the background tissue(5)wavesDPDWPhased arraybased on diffusefast Imaging ofresolution isCW lightunderadditional work needed to retrieveimager (6)photon densitybrain functionquestionablesource &investigationoptical properties from measuredwavesmodulationsignalFrequency-based ondetermineresolution & accuracydiode laserundercannot retrieve scatteringdomaindiffusion theoryopticalis questionableand intensityinvestigationcoefficient properly; unable toopticalpropertiesmodulatordetermine optical properties of thetomographybackground(7)(1) Watanabe et al. (1998). (2) Siegel et al. (1999). (3) Schweiger et al. (4) Cheng and Boas (1998). (5) Boas et al. (1997). Chen et al. (1998). Li et al. (2000). (6) Chance et al. (1998). (7) Pogue et al. (1997).CT: Computed TomographyTOAST: Time-resolved Optical Absorption and Scatter TomographyDPDW: Diffuse Photon Density Wave    1. Even though time-resolved techniques offer the potential for determining the optical properties of tissues, their reliance on high time-resolution measurements makes it a very challenging task to carry out experimental studies for validating the methodology, and to develop suitable bedside instrumentation.    2. Quite a few of the available techniques have been used only to study the differences between the absorption coefficients of the object and its surrounding medium (e.g. Cheng, x. and D. A. Boas, 1998: Diffuse optical reflection tomography with continuous-wave illumination. Opt. Express 3, No. 3, 118-123). It is insufficient to use solely the absorption coefficient to describe a biological tissue or an object embedded in such a tissue, since scattering can usually not be ignored in such a medium.    3. Most of the available tomographic optical imaging methods are focused on studying the optical properties (the absorption and scattering coefficients and the asymmetry factor or phase function) of an object that is embedded in a turbid medium, e.g. a tumor in healthy tissue, assuming that the optical properties of the background medium (healthy tissue) are known. These techniques cannot easily be used to study the optical properties of the turbid background medium (healthy tissue). However, accurate knowledge of the optical properties of the background medium is critical for success in biomedical imaging.
The above-described limitations of existing approaches clearly show that there is an urgent need to develop and provide reliable methods to determine both;                (1) the optical properties of healthy biological tissue, and (2) the location and optical properties of an object (such as a tumor) that is embedded in the healthy tissue.        